1. Field of the Invention
This invention relates to a control for an automatic defrost refrigerator and in particular to an electronically controlled defrost system in which intervals between defrost cycles are varied as a function of minimum compressor run time and of refrigerator usage as measured by door open time.
2. Description of the Prior Art
It is well known in a frost-free refrigerator to control the time interval between defrost cycles by means of a motor operated electromechanical timer means coupled across the motor operated compressor of the refrigerant system. Thus, in this arrangement, the compressor and defrost timer run and shut off simultaneously under control of the refrigerator compartment thermostat until a preset interval of compressor run time is reached, e.g. 71/2 hours, as determined by the timer. At this point, a switch on the timer removes the compressor motor and substitutes the defrost heaters. During the defrost cycle, typically thirty minutes, the timer continues to run until the defrost cycle is completed, at which point the timer switch reverts to the motor compressor circuit and the entire cycle as described above is repeated. U.S. Pat. No. 3,890,798 is representative of art teaching the use of a digital timer in place of the electromechanical timer to set the interval between defrost cycles as a function of compressor run time.
In U.S. Pat. No. 2,701,450, an electromechanical accumulator and door open sensor are shown for the purpose of providing demand defrost as a function of accumulated door open time. However, a variety of environmental conditions, as well as structural conditions of the refrigerator itself, can affect the timing of needed defrost cycles. This patent disclosure has no provision to take this into account except to provide for a manual override by which the user can accelerate or delay the timing of the initiation of the defrost cycles. U.S. Pat. No. 4,056,948 shows an arrangement for modifying the accumulation of compressor run time by a humidity controlled switch which operates to extend the time interval between defrost cycles in response to low humidity conditions in the refrigerator. This compensation must be manually selected, however, and does not take in account other factors which have an effect on the need for defrosting.
U.S. Pat. No. 2,781,641 discloses in FIG. 3 a defrost timing control circuit in which the timer is operated conjointly with the simultaneous occurrence of compressor run and door open conditions by means of door sensor and compressor run switches connected in series with the timer motor. Such an arrangement, however, does not allow for the possibility of initiating defrost independent of door opening time as might be required during very low usage periods or while the user is away on vacation. Moreover, there is no provision in this arrangement whereby initiation of the defrost cycle may be delayed from a fixed minimum time interval to a delayed time which is a measure of a fixed amount of actual door opening time. The serial connection of the door sensor and compressor switches requires that both be closed to operate the timer.